As documented in the Nextion Editor Guide, you can only change the attributes at run time that are shown green in the editor's attribute pane.
I deduce you are using an "intelligent" class device which permits x and y to be adjusted, but not w and h. The lower class devices don't allow changes to x and y either.
Not getting any errors or hangups of any sort, but I have noticed that other GUI commands don't work via serial either, such as draw, line, fill, etc. But changing properties of existing components is just fine (.txt, .val, etc). It's strange...
All I can suggest is to test with a bare-bones Nextion project and Arduino sketch to eliminate as many variables from the system as possible. If it still doesn't work, then post the sketch and HMI file on the forum.